Lately, there has been interest in fabricating laser diodes with several lasing stripes located on the same substrate. Because the semiconductor material used in laser diode manufacture is quite expensive, the lasing stripes usually share common electrical contacts in order to save space. Thus they have to operate simultaneously.
It is desirable, however, to operate the individual laser stripes independently of each other. For this purpose, the electrical contacts on the laser chip are delineated to correspond to individual laser stripes, and the whole chip is then bonded face-down on a carrier, which is made of a less expensive material, such as silicon. This is known in the art as `flip-chip` bonding.
The carrier, which is considerably larger than the laser chip, has a metal pattern mimicking the contact pattern of the laser stripes, and a set of bonding pads connected to the individual contacts. By applying electrical signals to the pads of the carrier, the laser stripes are made to operate independently of each other.
Generally, to control the light output of laser diodes, electronic feedback systems are utilized, which sense the laser's light output and adjust the drive current in such a way as to keep the light output constant and usually equal to some pre-set value. Some laser diode applications also require emission of light of precise wavelength. For this purpose, as well as heat removal in general, the laser diodes are mounted on thermoelectric cooler modules.
These modules are much larger than the laser diodes themselves, and often the laser is mounted on the module, rather than vice-versa. Furthermore, these modules are assembled of a large number of individual components, which makes them rather expensive.
The problems arise when the output of each laser stripe has to match the output of any other on the chip. The laser diodes in general, and the laser stripes in particular, heat up while being electrically driven during their operation. With time, their electro-optical characteristics also change.
For instance the so-called threshold current, which is the drive current at which lasing operation of the diode commences, tends to increase with age and also with temperature.
Also, due to manufacturing process variations, the individual laser diodes, even taken from the same substrate wafer, may have different electro-optical characteristics, such as efficiency (i.e. how much light is generated with a given drive current), and already mentioned threshold current.
In addition, the output wavelength of a laser stripe is affected by its temperature, which, in turn, depends on that stripe's drive current.
Without cooling, different drive currents will induce different temperature in the laser stripes, which will cause a shift both in their electrical characteristics, and also in the wavelength of the light each of them emits. Even addition of a common cooler does not solve this problem, as some of the stripes may be over-cooled, while others will remain relatively hot for their optimal operation.
The purpose of this invention is, therefore, to provide each laser stripe with its own cooling source to control its electrical and optical characteristics.
Another purpose of this invention is to provide a compact and inexpensive cooling module for laser diodes in general, whereby the module is incorporated, preferably along with its controlling circuitry, on the laser diode carrier.
Further advantages of the invention will become apparent as the following description proceeds.